1. Field of the Invention
The present invention relates to a manual transmission for a four-wheel drive vehicle capable of transmitting an engine power to both front and rear wheels of a vehicle.
2. Discussion of Related Arts
Generally, a manual transmission comprises an input Shaft inputting engine power through a clutch, a plurality of drive gears mounted on the input shaft, an output shaft and a plurality of driven gears constantly meshing with the respective drive gears, forming a plurality of shift gear trains between the input and output shafts. Further, the manual transmission has clutches such as synchromesh mechanisms for changing over the engagement of the respective gear trains between a power transmission condition and a power shut-off condition and when a driver operates a shift lever, the clutches are actuated to change over either of the gear trains to a power transmission condition.
The manual transmission for a four-wheel drive vehicle has a front drive shaft provided coaxially in a hollow output shaft and connected with a front final reduction gear apparatus or a front differential and a rear drive shaft connected with a rear final reduction gear apparatus or a rear differential through a propeller shaft. Further, the transmission has a power distribution apparatus or a transfer unit for distributing power between the front drive shaft and the rear drive shaft. The power distribution apparatus can change a power distribution ratio of front wheels to rear wheels of driving force, when drive wheels slip on a road surface with low friction coefficient or when there occurs a difference of rotation by more than a specified value between the front wheels and the rear wheels.
In a conventional manual transmission for a four wheel drive vehicle in which the output shaft is disposed below the input shaft, the power distribution apparatus is disposed coaxially with the output shaft at the rear of the output shaft. In thus layouted transmission, a large thrust load is applied from an orthogonal gear of the front final reduction gear to bearings for supporting the output shaft through the front drive shaft. As a result, not only the bearings are needed to be up-sized, a large power loss generates in the bearings due to the large thrust load applied thereto. Further, the up-sizing of the bearings itself generates a larger rotational loss. Hence, this type of the manual transmission has a difficulty in enhancing an efficiency of power transmission.
On the other hand, lubrication oil is accommodated in the transmission case in order to lubricate meshing surfaces of the gears and sliding surfaces of the shafts. When a vehicle travels, the lubrication oil is splashed by rotating gears and is supplied to required parts of the transmission. In case where the power distribution apparatus is coaxially disposed with the output shaft as described above, the power transmission apparatus is positioned at the lower part of the transmission. Hence, since the lubrication oil is agitated, while a large part of the power distribution apparatus is dipped in under the static oil level, a larger agitation resistance is exerted on the output shaft.
To solve this problem, Japanese Patent Application Laid-open No. Toku-Kai-Hei 10-166877 discloses a manual transmission for a four wheel drive vehicle in which an output shaft is coaxially disposed with a first input shaft connected to an engine and a second input shaft connected with the first input shaft through gears is arranged below the output shaft and a plurality of gear trains are provided between the output shaft and the second input shaft. The advantage of this transmission is that the power distribution apparatus can be disposed coaxially with the output shaft above the second input shaft and since the power distribution apparatus is located in a position higher than the oil level, the agitation resistance in the power distribution apparatus can be reduced.
However, in this type of the transmission, since a front drive shaft is incorporated in a hollow second input shaft, there always exists a rotational difference between the second input shaft and the output shaft, except when the first input shaft is directly connected with the output shaft in a fourth gear position. That is, since the front drive shaft slides around the second input shaft with a working load of gear applied to both shafts, a loss of power transmission is generated on the sliding surface.